Electric machines that have a stator and a rotor are known in different embodiments from the prior art. The windings in the stators and/or rotors of said electric machines are often formed by relatively thin individual wires, in particular of copper, which are inserted into the openings of a stator or rotor iron, the so-called slots, manually or with the aid of corresponding winding or pull-in machines. Typical diameters of the wires used here are approximately 2 mm.
Alternatively, it is also possible to use what are known as bar windings of copper, whereby individual bar segments are inserted into the slots instead of the individual wires. Said bar segments are shaped in such a way that they can be connected to one another at the ends thereof, in particular by welding, to form a bar winding arrangement, which forms a continuous winding of the stator and/or rotor. In the prior art, U- or V-shaped individual bar segments are often used in particular, said bar segments being reminiscent of hairpins due to their shaping and therefore often also being referred to as hairpins. Bar winding arrangements of this kind afford some advantages in comparison to the conventional individual wire windings. Whereas the manufacture of individual wire windings still requires many manual manufacturing steps in spite of a high degree of automation, bar winding arrangements can be produced in a fully automatic manner. Moreover, bar winding arrangements permit a better use of space of the slots, since bar segments having a rectangular cross section, which have a relatively large cross-sectional area, are generally used. When thinner individual wires are used, empty spaces always remain within the slots, even in the case of a tight geometric packing. Further packing losses arise as a result of an electrical insulation coating provided for the individual wires. Whereas slot filling factors with an order of magnitude of from approximately 30% to 50% arise in the case of windings of individual wires, slot filling factors of more than 80% can be achieved with the aid of bar winding arrangements. As a result, higher machine powers can be achieved with a smaller installation space.
More reliable electrical insulation between the bar segments themselves and between the bar segments and the stator or rotor iron is possible owing to the well-defined surfaces and the relatively large dimensions of the individual bar segments, in particular when using the hairpin-shaped bar segments (“hairpins”).
As is known from U.S. Pat. No. 8,330,318, which is incorporated by reference herein, when using substantially U-shaped bar segments, during assembly the individual segments can be inserted into the slots starting from the front side, with the result that it is possible to realize slots that are closed toward the rotor and are half-open to form an air gap, which can be achieved only with great difficulty or not at all in the case of conventional individual wire windings having a continuous wire.
Electric machines, in which bar winding arrangements based on hairpin-shaped bar segments are used, are of great interest, particularly in the automotive industry, due to the high energy density thereof and the possibility of automated manufacture. Examples of bar winding arrangements of this type are provided by DE 10 2010 036 428 A1, which is incorporated by reference herein, or US 2009/0140596 A1, which is incorporated by reference herein. As is known from U.S. Pat. No. 7,759,835 B2, which is incorporated by reference herein, or DE 10 2009 040 64 A1, which is incorporated by reference herein, instead of the U-shaped or V-shaped hairpin-like bar segments, which during assembly can be inserted into two slots, it is also possible to use shorter bar segments, which are each inserted into just one slot.
Bar winding arrangements having solid bar segments, which generally have a rectangular cross section, have considerable advantages compared to conventional individual wire windings in central key ratios, in particular at medium rotation speeds of the electric machine. However, in the case of high rotation speeds of the electric machine, in particular at rotation speeds of approximately 11000 rotations per minute and higher, the large cross sections of the bar segments give rise to relatively high power losses, which are caused by high-frequency effects.
In actual fact, a lower winding resistance and therefore lower losses should be able to be achieved by the higher slot filling factor that can be achieved by means of the bar winding arrangements. However, this is true only for direct current operation. However, at relatively high frequencies, as occur in a rotating electric machine, the losses actually increase. Although bar winding arrangements are clearly superior to individual wire windings in respect of their efficiency at low rotation speeds, the losses usually increase very greatly at high rotation speeds. The reason for this is high-frequency effects on account of the frequency, which increases with the rotation speed, both of the rotating field passing through the bar segments in the slot and the usually sinusoidal control current, which itself generates a magnetic field around itself. From a physical point of view, there are two mechanisms that create losses that increase, approximately quadratically, with the rotation speed. The skin effect and the proximity effect increase the effective resistance of the bar segments in such a way that relatively high losses are generated indirectly, since the electric current has to bridge a relatively high resistance. Furthermore, eddy currents in adjacent bar segments directly cause increased losses. Since the magnetic fields are largest in the vicinity of the slot openings, the two aforementioned effects are likewise greatest there. In order to counter said high-frequency effects, it is known from the prior art to segment the bar winding arrangements to a greater degree, by using a plurality of bar segments having smaller cross sections, for example, instead of bar segments having a large cross section. This is known, for example, from U.S. Pat. No. 6,956,313 B2. However, it has been shown that segmentation alone is not enough to reduce the power losses to a satisfactory degree.